<template>
  <div class="list">
    <ul>
      <li
        :ref="item"
        v-for="item of letters"
        @touchstart.prevent="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        @click="getChange"
      >
        {{item}}
      </li>
    </ul>
  </div>
</template>
<script>
  export default {
    name : "ListAlphabet",
    props : {
    cities : Object
  },
    data () {
      return {
        touchStatus : false,
        startY : 0,
        timer: null  //函数节流
      }
    },
    updated  () {
      this.startY = this.$refs['A'][0].offsetTop
    },
    computed : {
      letters (){
        const letters = [];
          for(let i in this.cities) {
            letters.push(i)
            }
          return letters;
      }
  },

    methods : {
      getChange (e) {
        const bar = e.target.innerText;
        this.$emit('change',bar);
      },
      handleTouchStart () {
        this.touchStatus = true
      },
      handleTouchMove (e) {
        if(this.touchStatus) {
            if(this.timer) {
              clearTimeout(this.timer)
            }
            this.timer = setTimeout(( )=>{
              const endY = e.touches[0].clientY - 95;
              const index = Math.floor((endY - this.startY)/20);
              this.$emit('change',this.letters[index])
            },16);
        }
      },
      handleTouchEnd () {
        this.touchStatus = false
      },
    }
  }
</script>
<style scoped>
  .list {
    display: flex;
    flex-direction: column;
    justify-content: center;
    position: absolute;
    top:95px;
    right: 0;
    bottom: 0;
    width: 20px;
    text-align: center;
  }
  .list ul li {
    line-height: 20px;
    font-size: 16px;

  }

</style>
